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Abstract

Background and purpose:

The pathology of the posterior acetabular lesions, so-called “contrecoup regions”, in femorocacetabular impingement (FAI) has not been elucidated fully. ¹⁸F-fluoride positron emission tomography/computed tomography (PET/CT) can visualize abnormal uptake caused by impingement. Therefore, we aimed to evaluate posterior acetabular uptake on PET/CT in FAI patients.

Patients and methods:

Patients with FAI who underwent ¹⁸F-fluoride PET/CT between October 2014 and October 2016 were retrospectively evaluated. The maximum standardized uptake value (SUV_max) in the posterior acetabulum was evaluated. The mean SUV_max of FAI with cam morphology (the cam group) was compared with that of FAI with pincer morphology (the pincer group). In addition, the numbers of cases with SUV_max ≥ 6 and SUV_max < 6 in each group were evaluated. The entire study cohort was also grouped according to SUV_max, and the mean α and center edge angles were evaluated.

Results:

In total, 41 hips were analyzed (34 hips in the cam group and 7 in the pincer group). The mean SUV_max of the cam group (11.2 ± 7.4) was significantly higher than that of the pincer group (4.9 ± 1.9) (p < 0.01). The incidence of cases with SUV_max ≥ 6 in the cam group was significantly high (p < 0.01). In the overall cohort, the mean α angle of the SUV_max ≥ 6 group was significantly higher than that of the SUV_max < 6 group (p < 0.01).

Conclusion:

Evaluation of posterior acetabular uptake suggests an association between cam morphology and increased posterior acetabular uptake.

Keywords

contrecoup injury femoroacetabular impingement positron emission tomography

Introduction

Femoroacetabular impingement (FAI) is a widely recognized hip disorder that is associated with a range of symptoms such as hip pain, clicking, and a restricted range of movement. Bony impingement between the anterior acetabular rim and the femoral head–neck junction is an important pathogenic consideration in patients with FAI and has been shown in many studies using techniques such as computer simulation, magnetic resonance imaging (MRI), and finite element analysis (FEA).^1

–7 However, few studies have evaluated posterior acetabular lesions, and the associated clinical implications therefore remain unclear.^8

–11 A greater understanding of the mechanism of posterior acetabular lesion development may be essential to fully understand the pathology of FAI.

The technique of ¹⁸F-fluoride positron emission tomography (PET) is a novel method of bone imaging that reflects osteoblast activity, and abnormal uptake on ¹⁸F-fluoride PET has been seen in patients with FAI.¹² This abnormal uptake is presumed to be caused by mechanical stress of impingement. Recent technological advances in PET combined with computed tomography (CT) imaging have allowed detailed three-dimensional morphological information to be obtained. Therefore, ¹⁸F-fluoride PET/CT is now capable of visualizing abnormal uptake caused by impingement as well as precise locational information. Using this technique, the precise location of abnormal uptake in the femoral neck in FAI cases was previously clarified.¹³ We speculated that posterior acetabular uptake on PET/CT may reflect a so-called “contrecoup region”, which has previously been described as a chondral injury in the posteroinferior acetabulum in pincer-type FAI cases.^8,9 The purpose of the present study was to evaluate posterior acetabular uptake on ¹⁸F-fluoride PET/CT in patients with different types of FAI. We hypothesized that ¹⁸F-fluoride PET/CT could be used to visualize posterior acetabular uptake in patients with FAI, the strength of which may vary with FAI type.

Materials and methods

Ethical permission was obtained for this study from Yokohama City University ethical committee (number B140508033). All patients provided written informed consent for participation in the study.

Patient

Between October 2014 and October 2016, we performed ¹⁸F-fluoride PET/CT on a total of 198 hips in 169 patients with a complaint of hip pain. The medical records of those patients were retrospectively reviewed. The analysis included the following types of FAI: cam (center edge (CE) angle ≥ 25° and α angle ≥ 55°), pincer (CE angle ≥ 40°, or CE angle ≥ 30° and acetabular roof obliquity ≤ 0°, or CE angle ≥ 25°and positive crossover sign), combined (both cam and pincer morphologies), and dysplastic developmental hip (DDH) with cam morphology (CE angle < 25° and α angle ≥ 55°). FAI was diagnosed using these radiographic parameters and the positive anterior impingement test. Joints that had undergone total hip arthroplasty (THA) or osteotomy were excluded from the analysis, as were hips showing signs of osteonecrosis (ON), osteoarthritis (OA) of Tönnis grade ≥ 2, or a minimum joint space < 2 mm. Cases that could not be assigned to one of the four FAI types and patients with DDH without cam morphology, those with painful hips of unknown cause, and those with other disorders (including infection, tumors, arthritis with collagen disease, Perthes disease, and rapidly destructive cox arthropathy) were also excluded.

PET/CT analysis

¹⁸F-fluoride PET/CT was performed using Celesteion™ (Toshiba Medical Systems Corporation, Tochigi, Japan). Patients received an intravenous infusion of ¹⁸F-fluoride (185 MBq) dissolved in 10 ml of 0.9% saline; scanning was performed 40 min after infusion. To determine the exact anatomical location of the maximum standardized uptake value (SUV_max), axial PET images were coregistered and fused with corresponding CT images. The region of interest for posterior acetabular uptake was defined as follows: a circle approximating the contour of the femoral head was drawn on the axial PET/CT image. The images in which the location of uptake was facing the posterolateral quadrant of the circle were evaluated to determine the SUV_max (Figure 1). The highest SUV_max in these images was adopted as the SUV_max.

Figure 1.

The ROI for posterior acetabular uptake. A circle relating to the contour of the femoral head was drawn on the axial image of PET/CT images. The images on which the location of uptake was facing the posterolateral quadrant of the circle were evaluated for ROI to determine SUV_max. Red arrow indicates posterolateral range of one-fourth of the circle, and white arrow indicates the location of uptake. ROI: region of interest; PET/CT: positron emission tomography/computed tomography; SUV_max: maximum standardized uptake value.

Statistical analysis

Normality of continuous data was assessed using the Kolmogorov–Smirnov test. We compared the mean SUV_max of FAI cases with cam morphology (the cam group) including those in cases of cam-type, combined-type, and DDH with cam morphology with those of FAI with pincer morphology (the pincer group) using the unpaired t-test. For nonparametric data, the Mann–Whitney U-test was used to compare between the two groups. Each group was further subdivided into two groups according to SUV_max ≥6 or <6, and the number of cases of SUV_max ≥6 and <6 in each group was analyzed using Fisher’s exact test. In addition, the entire cohort was also grouped on this basis, and the mean α angles and CE angles of the SUV_max ≥6 and the <6 groups were analyzed using the unpaired Student’s t-test. The values of p were two-sided, and p < 0.05 was considered statistically significant. All statistical analyses were performed with EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria).

Results

All of the patients who underwent PET/CT in the study period agreed to participate in this study. Cases were excluded due to THA (n = 106), osteotomy (n = 12), ON (n = 7), OA (n = 6), DDH without cam morphology (n = 5), hip pain of unknown cause (n = 5), and other disorders (n = 7). In addition, nine hips without cam and pincer morphologies, despite clinically suspected FAI were excluded. Therefore, 41 hip joints (22 right and 19 left) were included in the analysis (11 cam-type, 7 pincer-type, 11 combined-type, and 12 DDH with cam morphology). Of the 41 patients, 23 were female and 18 were male with a mean age of 44.9 years (range 16–66 years). Figure 2 shows a representative case of cam-type FAI with posterior acetabular uptake. On the axial image of the PET/CT (Figure 2(c)), marked uptake is seen in the posterior acetabulum.

Figure 2.

A representative case of cam-type FAI in a 63-year-old female. (a) Anterior–posterior view X-ray shows 28° CE angle. (b) Cross-table lateral view X-ray shows 80° α angle. (c) Axial PET/CT image shows marked uptake in the right posterior acetabulum with an SUV_max of 17.8. FAI: femoroacetabular impingement; CE: center edge; PET/CT: positron emission tomography/computed tomography; SUV_max: maximum standardized uptake value.

The mean SUV_max of the cam group (n = 34) and that of the pincer group (n = 7) were 11.2 ± 7.4 and 4.9 ± 1.9, respectively. The mean SUV_max of the cam group was significantly higher than that of the pincer group (p < 0.01) (Table 1).

Table 1.

The mean SUV_max of each FAI group.^a

	Mean SUV_max
FAI with cam morphology (n = 34)	11.2 ± 7.4
Pincer-type FAI (n = 7)	4.9 ± 1.9

SUV_max: maximum standardized uptake value; FAI: femoroacetabular impingement.

^a p < 0.01: Mann–Whitney U test.

The distribution of cases with SUV_max ≥ 6 and SUV_max < 6 in each FAI group was significantly different (p < 0.01), and the incidence of cases with SUV_max ≥ 6 in the cam group was especially high (Table 2).

Table 2.

The number of cases with SUV_max ≥ 6 and SUV_max < 6 in each FAI group.^a

	FAI with cam morphology	Pincer-type
SUV_max ≥ 6, n	30	2
SUV_max < 6, n	4	5

SUV_max: maximum standardized uptake value; FAI: femoroacetabular impingement.

^a p < 0.01: Fisher’s exact test.

In the overall cohort, the SUV_max ≥ 6 group comprised 32 hips and the SUV_max < 6 group comprised 9 hips. The mean α angle of the SUV_max ≥ 6 group was significantly higher than that of the SUV_max < 6 group (64 ± 9.5° vs. 53 ± 11.6°, respectively; p < 0.01). The mean CE angle of the SUV_max ≥ 6 group was 29 ± 9.3° and that of the SUV_max < 6 group was 33 ± 5.8°, although this difference was not statistically significant (p = 0.26; Table 3).

Table 3.

The mean α and CE angles of SUV_max ≥ 6 and SUV_max < 6 groups.

	SUV_max ≥ 6 group (n = 32)	SUV_max < 6 group (n = 9)	p Value
Mean α angle	64	53	<0.01
Mean CE angle	29	33	0.26

CE angle: center edge angle; SUV_max: maximum standardized uptake value.

Discussion

In this study, the mean SUV_max of the cam group was significantly higher than that of the pincer group. In addition, the incidence of cases with SUV_max ≥ 6 in the cam group was significantly high. In the overall cohort, the mean α angle of the SUV_max ≥ 6 group was significantly higher than that of the SUV_max < 6 group, although the mean CE angle did not differ between the two groups. These results indicate that posterior acetabular uptake is associated with cam morphology rather than pincer morphology.

While previous reports evaluated posterior acetabular lesions using motion capture,¹¹ MRI,^14,15 four-dimensional volume CT,¹⁶ surgical dislocation,⁸ and arthroscopic findings,^17,18 a key strength of the current study was the use of ¹⁸F-fluoride PET/CT. Bony impingement is a significant pathology in FAI and results in chondral/labral injuries, which may induce a local tissue reaction prior to radiographic changes, as demonstrated in early-stage OA.¹⁹ Uptake of ¹⁸F-fluoride reflects regional blood flow and new bone formation.²⁰ Thus, ¹⁸F-fluoride PET imaging can be utilized to evaluate functional abnormalities that occur at the cellular level. In a previous study, the mechanical stress by FEA was compared with ¹⁸F-fluoride PET uptake in dysplastic hip joints, which demonstrated that the location of increased ¹⁸F-fluoride PET uptake was consistent with the site of concentrated equivalent stress. At the same time, the actual value of equivalent stress correlated significantly with ¹⁸F-fluoride PET uptake.²¹ Furthermore, another previous study revealed that in 82% of FAI cases with cam morphology, the SUV_max region on PET/CT was in concordance with the impingement region by computer simulation.¹³ Therefore, we presume that bony impingement induces repetitive mechanical stress and affects the bone remodeling status in subchondral bone in FAI cases and that ¹⁸F-fluoride PET/CT can detect it and additionally provide precise anatomical information. In the current study, we divided the cases into cohorts according to SUV_max ≥6 or <6. This is because the average SUV_max in a control group has been reported to be less than 6, while the average SUV_max of the group with hip OA and that of the group with hip pain were more than 6 in a study evaluating hip OA cases with ¹⁸F-fluoride PET.¹⁹ Although we adopted the SUV_max threshold of OA cases, the SUV_max of FAI cases should also be determined.

In a previous study, Pfirrmann et al. analyzed the magnetic resonance arthrographic results of 50 FAI cases and observed more severe cartilage damage at the posterior and posteroinferior aspects of the acetabulum in cases of pincer FAI than in cases of cam FAI.¹⁴ Ganz et al. directly inspected the femoral head and the acetabulum during the surgical dislocation of hip joints, demonstrating the mechanism of contrecoup region development; in patients with pincer-type FAI, flexion causes the femoral head to be levered out posteriorly, creating a lesion in the posteroinferior acetabular region.^8,9 This kinematic mechanism of contrecoup region development provides a rational explanation that may be applicable to conditions other than pincer-type FAI. Here, we speculated that both cam and pincer morphology may induce contrecoup posterior lesions. A systematic review conducted by Canham et al. evaluated studies of patients with FAI and hip instability, demonstrating that the rates of cam and pincer morphologies in patients with instability events were 74% and 64%, respectively.²² Furthermore, they observed that the rate of cam impingement in patients with instability was considerably higher than the prevalence of asymptomatic cam lesions in the general population (74% vs. 37%, respectively), while the rate of pincer impingement was similar to that of asymptomatic pincer lesions (64% vs. 67%, respectively).²³ The authors concluded that cam impingement may predispose hip joint to instability to a greater extent than pincer impingement. Their results and theory support our results; the mean SUV_max of the patients with cam morphology (i.e. the cam group including the cam, combined, and DDH with cam morphology groups) was significantly higher than that of the patients with pincer morphology (i.e. the pincer group).

Interestingly, in the current study, the mean SUV_max of the DDH with cam morphology group was 13.4 ± 10.9, which is a relatively high value among the four types, although it was not statistically significant. The combination of joint instability and cam morphology may induce more severe posterior acetabular uptake than isolated cam morphology. In a study by Charbonnier et al., motion capture and MRI techniques were used to evaluate 11 ballet dancers, demonstrating that, in all evaluated hips, subluxations always visually correlated with impingements between the proximal femur and the acetabular rim. In addition, the author reported that, in more than 80% of the dancers’ hips, degenerative labral lesions and acetabular damages were diagnosed in the superior and posterosuperior regions of the acetabular rim.¹¹ These results indicate that the posterior instability of the femoral head induced by the impingement between the anterior acetabular rim and femoral head–neck junction (i.e. the contrecoup mechanism) may be a factor in the development of posterior acetabular lesions. To evaluate hip instability in the clinical setting, the apprehension test and arthroscopic inspection are utilized.^24,25 However, neither were performed in the current study, and this is an important limitation. Further studies should be conducted to investigate the correlation between abnormal uptake and arthroscopic findings as well as whether the contrecoup region occurs in DDH cases.

Conclusion

In this study, we evaluated posterior acetabular uptake in patients with FAI using ¹⁸F-fluoride PET/CT. The results suggested that cam morphology is associated with higher levels of posterior acetabular uptake on ¹⁸F-fluoride PET/CT.

Footnotes

Acknowledgements

The authors appreciate the contribution of the Department of Radiology in Yokohama City University in performing all imaging modalities.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Takayuki Oishi

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Posterior acetabular uptake on 18 F-fluoride positron emission tomography/computed tomography reveals a putative contrecoup region in patients with femoroacetabular impingement

Abstract

Background and purpose:

Patients and methods:

Results:

Conclusion:

Keywords

Introduction

Materials and methods

Patient

PET/CT analysis

Statistical analysis

Results

Discussion

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References